British Airways will tomorrow reveal the full extent of a disastrous three months in which it was hit by a double whammy of a volcanic ash cloud from Iceland and cabin crew strikes grounding planes.

Analysts said they expected the losses to be "very bad", which could put further pressure on Willie Walsh, the airline's chief executive, to reach an agreement with unions to end the long-running dispute.

Talks are being held at the conciliation service Acas on Monday. Union sources said that they were keen to resolve the dispute with BA but they are understood to be pessimistic about the chances of a breakthrough taking place.

The Unite union has planned strike meetings with its officials next week to discuss any progress made at the talks and whether to take further industrial action if the impasse continues.

This month, two-thirds of Unite members who turned out voted to reject BA's "final" pay offer of a 6% rise over two years. BA claimed that the low turnout – only about half Unite's members voted – showed that support for the union's stance was waning. If Unite presses ahead with a strike ballot, industrial action could take place again as soon as early September.

The airline was forced to ground dozens of flights during seven days of volcanic ash in April. Walsh, along with other airline chief executives, was heavily critical of the Civil Aviation Authority, which ordered planes to be grounded while tests were carried out to gauge the effect of ash on engines. There were also 23 days of strikes in May and June by cabin crew.

Howard Wheeldon, an analyst at BGC Partners, said of the forthcoming first-quarter results: "There is no reason to think they won't be anything other than very bad."

The City will also be keen to see how future bookings have been affected by the continued threat of industrial action. BA has already lost an estimated £120m from the volcanic disruption and more than £150m from the wave of strikes over cost-cutting and the removal of travel concessions from union members who went on strike, as well as disciplinary issues.

When it released annual figures in May, BA said that market conditions were starting to improve and that it hoped to break even before tax for the year.

Unite today said 75% of crew at Heathrow has lost staff travel concessions, which proved how many had been on strike, in contrast to company claims that fewer employees were joining the action.

Walsh has pledged to run 100% of long-haul services during any future industrial action and has been hiring extra cabin crew in an attempt to keep planes flying.